As a measure against global warming, solar photovoltaics become popular around the world. The total production of solar cells in the world in 2005 was 1, 759 MW, which is an increase by 147% over the previous fiscal year. At present, the popular solar cell is crystal solar cells, and solar cells using single crystal silicon or polycrystalline silicon account for the large part of the production. These solar cells use a silicon wafer as a base, which is formed by slicing a large-sized silicon ingot into a circle.
In a crystal solar cell using silicon as a material, a thickness of a silicon wafer which is sufficient to absorb sun light is about 10 μm. However, an actual silicon wafer has a thickness of about 200 μm to 300 μm. This means that a silicon wafer which is used for the crystal solar cell is more than or equal to ten times as thick as the silicon wafer needed to be for photoelectric conversion and it is difficult to say that a silicon wafer is used effectively. In extreme terms, most of silicon wafers only functions as a structure for maintaining the shape of the solar cells.
As production of solar cells increases, shortage in supply of polycrystalline silicon, which is a material of silicon, and steep price rise of silicon wafers become problems in industry. The production of polycrystalline silicon in 2007 is expected to be about 36 thousand tons; however, polycrystalline silicon which is demanded for semiconductors (LSI) is 25 thousand tons or more, and for solar cells is 20 thousand tons or more, whereby shortage in supply of polycrystalline silicon is excepted to be about 10 thousand tons. Furthermore, it is expected that such a shortage in supply will continue.
However, development of crystal thin film silicon solar cells has been proceeding in anticipation of these circumstances. For example, a method for manufacturing a silicon thin film solar cell in which a crystalline silicon film is deposited over a substrate by using a VHF which is 27 MHz or more and pulse modulating the VHF, is disclosed (see Patent Document 1: Japanese Published Patent Application No. 2005-50905). Further, a technique for controlling plasma treatment conditions to optimize addition of a dopant to crystal grains and crystal grain boundaries when a thin film polycrystalline silicon film is formed by a plasma CVD method over a special electrode called a texture electrode which has minute unevenness on its surface, is disclosed (see Patent Document 2: Japanese Published Patent Application No. 2004-14958).